Lanthanum cerium aluminate phosphor and an electrical discharge device containing the same

ABSTRACT

Lanthanum cerium aluminate phosphor compositions are disclosed wherein the formula for the phosphor can be characterized as follows: 
     
         La.sub.1-x Ce.sub.x Al.sub.y O.sub.(3/2)(y+1) 
    
     wherein 0&lt;x&lt;1 and 11≦y≦13.8.

This is a continuation-in-part of copending application Ser. No.07/312,326 filed on Feb. 16, 1989, abandoned which is a continuation ofapplication Ser. No. 06/560,296 filed on Dec. 12, 1983, abandoned.

FIELD OF THE INVENTION

This invention relates to phosphor compositions. More particularly, itrelates to lanthanum cerium aluminate phosphor compositions, a processfor making the phosphor compositions, a lamp provided with the phosphorcompositions, and a phosphor blend incorporating the phosphorcompositions.

BACKGROUND OF THE INVENTION

Sommerdijk and Stevels in an article entitled "The Behavior ofPhosphors, with Aluminate Host Lattices," Philips Technical Review,volume 37, 221-233 (1977), discuss lanthanum aluminates and ceriumaluminates. The article reports the existence region for the ceriumaluminates and lanthanum aluminates with the formulae (La₀.86O₀.14)Al₁₁.9 O₁₉ and (Ce₀.86 O₀.14)Al₁₁.9 O₁₉, respectively, as being"fairly narrow". The article further includes a discussion of a seriesof compositions represented by the formula (La_(1-x) Ce_(x))₀.86 Al₁₁.9O₁₉.14.

In a later article entitled "Ce³⁺ Luminescence in Hexagonal AluminatesContaining Large Divalent or Trivalent Cations," J. Electrochem Soc.125, 588-594 (1978), Stevels discusses the luminescent properties of the(La_(1-x) Ce_(x))0.86Al₁₁.90 O₁₉.14 series. The luminescence spectra ofthe compositions are reported to display two bands: one having a maximumin 330-350 nm region and the other with a maximum in the 410-430 nmregion.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, there is provided aluminescent phosphor composition consisting essentially of lanthanumcerium aluminate phosphors represented by the formula

P0378 12/14/83 560296 07-2344 2 101 300.00CH P0379 12/14/83 56029607-2344 2 102 60.00CH P0380 12/14/83 560296 07-2344 2 103 10.00CH

    La.sub.1-x Ce.sub.x Al.sub.y O.sub.(3/2) (y+1)

wherein 0 <x< 1 and 11 ≦y≦ 13.8.

In another aspect of the invention, a process for preparing lanthanumcerium aluminate luminescent phosphor compositions comprises blendinglanthanum (III) oxide, cerium (IV) oxide, and aluminum hydroxidetogether in the desired ratios to form a relatively uniform admixture,and then firing the admixture in a reducing atmosphere at an elevatedtemperature.

In accordance with a further aspect of the invention, a method forpreparing lanthanum cerium aluminate luminescent phosphor compositionscomprises coprecipitating lanthanum and cerium from solution as eitheroxalates or carbonates in the presence of aluminum oxide to form aprecipitate containing lanthanum, cerium, and aluminum in the desiredratios; heating the precipitate; and firing the heated precipitate in areducing atmosphere at an elevated temperature.

In another aspect of the invention, there is provided a fluorescent lamphaving a coating of the above-described lanthanum cerium aluminatephosphor composition on the inner surface of the envelope.

In a further aspect of the invention, there is provided a white emittingphosphor blend containing a blue-emitting phosphor wherein the blueemitting phosphor is a phosphor composition of this invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the emission spectra of the known phosphorcompositions CeAl₁₃.5 O₂₁.75, CeAl₁₂.5 O₂₀.25, and CeAl₁₁ O₁₈.

FIG. 2 illustrates the emission spectra of phosphor compositions of thisinvention represented by the formulae La₀.70 Ce₀.30 Al₁₃.5 O₂₁.75,La0.70Ce₀.30 Al12.5O₂₀.25, and La₀.70 Ce₀.30 Al₁₁ O₁₈.

FIG. 3 illustrates the emission spectra of phosphor compositions of thisinvention represented by the formulae La₀.90 Ce₀.10 Al₁₃.5 O₂₁.75,La₀.90 Ce₀.10 Al₁₂.5 O₂₀.25, and La₀.90 Ce₀.10 Al₁₁ O₁₈.

FIG. 4 illustrates the emission spectra of phosphor compositions of thisinvention represented by the formulae La₀.99 Ce₀.01 Al₁₃.5 O₂₁.75,La₀.99 Ce₀.01 Al₁₂.5 O₂₀.25, and La₀.99 Ce₀.01 Al₁₁ O₁₈.

For a better understanding of the present invention together with otherand further objects, advantages, and capabilities thereof, reference ismade to the following disclosure and appended claims in connection withthe above described drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

This invention relates to a luminescent phosphor composition consistingessentially of lanthanum cerium aluminate phosphors represented by theformula

    La.sub.1-x Ce.sub.x Al.sub.y O.sub.(3/2) (y+1)

wherein 0 <x<1 and 11≦y ≦13.8. These lanthanum cerium aluminatephosphors possess a structure similar to the β-alumina andmagnetoplumbite structures. The range of y, 11≦y≦13.8, is that of aluminescent magnetoplumbite/62 -alumina single phase solid solution.These compositions are further characterized by "ultraviolet" emissionpeaking at about 300 to 370 nanometers and "blue" emission peaking atabout 400 to 500 nanometers. The values of x and y further determinewhether the emission from these materials under 254 nanometer excitationwill be predominantly in the blue region of the spectrum orpredominantly in the ultraviolet region of the spectrum. As used herein,"predominantly" means "having the greater integrated relativeintensity". Compositions for which y has values less than 11 and greaterthan 13.8 also emit under 254 nanometer excitation in the blue andultraviolet regions of the spectrum. However, these compositions do notfall within the bounds of the single phase solid solution region. Inthese compositions, one or more nonluminescent phases is produced inaddition to the single luminescent phase compositions.

FIG. 1 shows emission spectra at room temperature for cerium aluminatecompositions under 254 nanometer excitation. For purposes of discussion,these cerium aluminate compositions will be represented by the formulaLa_(1-x) Ce_(x) Al_(y) O.sub.(3/2) (y+1) wherein x =1. FIG. 1 whichrepresents the spectra for compositions wherein x =1 is included forpurposes of comparison only as such cerium aluminate compositions areknown. FIGS. 2 through 4 show emission spectra at room temperature forlanthanum cerium aluminate phosphor compositions of this invention under254 nanometer excitation. FIGS. 1, 2, 3 and 4 display the spectra forthe compositions La_(1-x) Ce_(x) Al_(y) O.sub.(3/2) (y+1) wherein x =1,0.3, 0.1, and 0.01, respectively. Each Figure shows spectra for aspecified fixed value of x wherein individual curves are for y =13.5,12.5 and 11.0.

The spectra show that for compositions with a fixed cerium and lanthanumcontent the relative intensity of the ultraviolet band increases as thealuminum content is increased. Of the compositions whose emissionspectra are depicted in FIGS. 1 through 4, the maximum ultravioletrelative intensity is exhibited by compositions wherein there are 13.5atoms of aluminum per formula unit.

In comparing the spectra for composition sequences with a fixed aluminumcontent per formula unit, the relative intensity of the emissions in theultraviolet region of the spectrum increases as the cerium content isdecreased and the lanthanum content is correspondingly increased.Conversely, the relative intensity of the blue band increases as ceriumis substituted for lanthanum.

Table 1 summarizes the relative intensities (relative peak heights) ofthe ultraviolet emission band (300-370nm) and the blue emission band(400-500nm) for various compositions of the present invention whereinthe aluminum content is 13.5 atoms of aluminum per formula unit. InTable I, the greatest spectral intensity in the range 300-500 nm isdefined to be 100% for each composition. Table I does not compareabsolute intensities between compositions.

                  TABLE I                                                         ______________________________________                                        NORMALIZED PEAK HEIGHTS FOR INDIVIDUAL                                        LANTHANUM CERIUM ALUMINATE COMPOSITIONS                                       COMPOSITION      300-370 nm                                                                              400-500 nm                                         ______________________________________                                        La.sub.0.99 Ce.sub.0.01 Al.sub.13.5 O.sub.21.75                                                100%       5%                                                La.sub.0.95 Ce.sub.0.05 Al.sub.13.5 O.sub.21.75                                                100%      10%                                                La.sub.0.90 Ce.sub.0.10 Al.sub.13.5 O.sub.21.75                                                100%      20%                                                La.sub.0.70 Ce.sub.0.30 Al.sub.13.5 O.sub.21.75                                                100%      80%                                                La.sub.0.50 Ce.sub.0.50 Al.sub.13.5 O.sub.21.75                                                 85%      100%                                               La.sub.0.30 Ce.sub.0.70 Al.sub.13.5 O.sub.21.75                                                100%      85%                                                CeAl.sub.13.5 O.sub.21.75                                                                       92%      100%                                               ______________________________________                                    

Ultraviolet emissions ranging from significant to predominant areobserved for the compositions listed in Table 1. It is further notedthat the relative intensity, as defined above, of the ultraviolet bandis unaffected by the appearance of the blue band as the cerium contentis increased, ranging from 85% to 100%. Moreover, the quantumefficiency, based on integrated intensity and absorption remainsessentially constant, changing only a few percent over the range ofphosphor compositions in this invention.

The lanthanum cerium aluminate luminescent compositions of thisinvention represented by the formula

    La.sub.1-x Ce.sub.x Al.sub.y O.sub.(3/2) (y+1)

wherein 0<x≦0.10 and 13 ≦y≦13.8 have application as ultraviolet emittingphosphors. These compositions are characterized by predominantlyultraviolet peak emission. These predominantly ultraviolet emittingcompositions are suitable for use in uv-emitting fluorescent lamps.

The lanthanum cerium aluminate phosphor compositions of this inventionrepresented by the formula

    La.sub.1-x Ce.sub.x Al.sub.y O.sub.(3/2) (y+1)

wherein 0.10 <x <1.0 and 11≦y ≦12.5, and preferably 0.10 x <1.0 and 11≦y ≦12, have application as blue-emitting phosphors. These compositionsare characterized by predominantly blue peak emission. Application forthese predominantly blue-emitting compositions includes, use as the blueemitting phosphor in a white-emitting phosphor blend containing ablue-emitting phosphor for use in fluorescent lamp of high colorrendering index.

A process for preparing the luminescent phosphor composition of thisinvention consisting essentially of lanthanum cerium aluminate phosphorsrepresented by the formula La_(1-x) Ce_(x) Al_(y) O.sub.(3/2) (y+1)wherein 0<x<1.0 and 11.0≦y ≦13.8 comprises the sequential steps ofblending lanthanum (III) oxide, a cerium (IV) oxide, and aluminumhydroxide, together in the desired ratios to form a relatively uniformadmixture; precalcining the admixture in air at a temperature of about850° C. for about one hour; firing the product of the precalcining stepin a reducing atmosphere at an elevated temperature for about 3 to about4 hours; pulverizing the product of the firing step; firing the productof the pulverizing step in a reducing atmosphere at an elevatedtemperature and pulverizing the product of the second firing step. Theprocess may further include the steps of firing the product of thesecond pulverizing step in a reducing atmosphere at an elevatedtemperature to form a fired material and pulverizing the fired material.Examples of a suitable reducing atmosphere for use in the above processfor the preparation of the lanthanum cerium aluminate phosphorscompositions of this invention include hydrogen gas or a mixture ofgases consisting of at least 5 volume percent hydrogen with the balanceconsisting of an inert gas such as nitrogen or argon. The reducingatmosphere can be wet or dry.

The firing steps can be carried out at temperatures from about 1600° toabout 1800° C. The preferred firing temperature to be used in the aboveset forth process is from about 1750° to about 1775° C., the morepreferred firing temperature being about 1750° C.

Another method for preparing the lanthanum cerium aluminate luminescentphosphor composition of this invention involves forming a precipitatecontaining lanthanum, cerium, and aluminum in the desired ratios,heating the precipitate to the corresponding oxide and converting theoxide to the luminescent phosphor composition of this invention byfiring the oxide in a reducing atmosphere at an elevated temperature.Precipitation agents that can be used in this method include oxalic acidor ammonium carbonate. The precipitation method employing an oxalic acidprecipitation agent comprises the sequential steps of dissolvinglanthanum (III) oxide, and cerium (III) nitrate hexahydrate inpredetermined proportions as desired in said phosphor composition in hotacid and diluting the solution with water. The diluted solution is nextheated to a temperature of about 90° C. Gamma-aluminum oxide, in apredetermined proportion as desired in said phosphor composition, iscarefully added to the 90° C. solution. Solid oxalic acid is next addedto the heated solution, with stirring, in amounts at least sufficient tofully precipitate the lanthanum and cerium ions. The solution is allowedto cool to ambient temperature and let stand undisturbed for a period oftime sufficient for the precipitation reaction to occur. The precipitateis collected, dried at a temperature of about 140° C., cooled to ambienttemperature and pulverized. The pulverized material is next fired in areducing atmosphere at a temperature of from about 1600° to about 1800°C. for about 3 to about 4 hours, cooled to ambient temperature, andpulverized.

Examples of a suitable reducing atmosphere for use in the firing stepinclude hydrogen or a mixture of gases consisting of at least 5 volumepercent hydrogen with the balance consisting of an inert gas, such asnitrogen or argon. This method may optionally include additional firingand pulverizing steps.

The precipitation method employing an ammonium carbonate precipitationagent comprises the sequential steps of dissolving lanthanum (III) oxideand cerium (III) nitrate hexahydrate in predetermined proportions asdesired in said phosphor composition in hot acid; diluting the solutionwith water; adding γ-aluminum oxide, in a predetermined proportion asdesired in said phosphor composition, to the solution; neutralizing thesolution with ammonium carbonate; adding a small amount of carbonate inexcess of the amount needed to neutralize the solution; stirring thesolution; allowing the solution to stand undisturbed for a period oftime sufficient for the precipitation reaction to occur; collecting theprecipitate; drying the precipitate at a temperature of about 140° C.;and pulverizing the precipitate. The pulverized precipitate is nextfired in reducing atmosphere at a temperature of from about 1600° toabout 1800° C. for about 3 to about 4 hours, cooled to ambienttemperature and pulverized.

Examples of a suitable reducing atmosphere for use in the firing stepinclude hydrogen or a mixture of gases consisting of at least 5 volumepercent hydrogen with the balance consisting of an inert gas such asnitrogen or argon. This method may further include additional firing andpulverizing steps.

The fluorescent lamps of the present invention can have the structure asshown, for example, in U.S. Pat. Nos. 2,151,496; 3,424,605; 3,424,606;or 3,435,271; the figures of which, and the portions of thespecification corresponding thereto, are incorporated herein byreference to the extent necessary to complete this specification. Suchfluorescent lamps consist of a hermetically sealed tubular glassenvelope coated on its inside surface with a powdered phosphor coatingcomprising the luminescent phosphor composition of this invention. Thebases at each end of the tubular envelope support electrical leads whichare electrically connected to electrodes at each end of the lamp. Thespaced electrodes can be coated with electron-emission promotingmaterials, such as mixtures of oxides containing a barium oxide, tofacilitate operation of the lamp. During operation, a mercury dropletmaintained within the sealed envelope is vaporized thereby causing thecharacteristic mercury discharge. Except for the nature of the phosphorcoating, which is the subject of the present invention, construction ofthe lamp is conventional.

An important feature of the phosphor composition of this invention isits resistance to degradation during the lehring (air baking) step oflamp fabrication. As shown in Table II, cerium aluminate undergoesapproximately a 23% reduction in emission during a five-minute air bakeat 600° C., whereas for lanthanum cerium aluminate the degradation isless. The degradation decreases monotonically with increasing lanthanumcontent and with decreasing exposure to air bake.

                  TABLE II                                                        ______________________________________                                        DEPENDENCE OF EMISSION DEGRADATION ON                                         LANTHANUM SUBSTITUTION IN La.sub.1- x Ce.sub.x Al.sub.11 O.sub.18             FOR AIR BAKING AT 600° C.                                                       Percent Emission                                                                             Percent Emission                                               Intensity Remaining                                                                          Intensity Remaining                                   x        After 5 min. Air Bake                                                                        After 1 hr. Air Bake                                  ______________________________________                                        1.0      77             24                                                    0.5      85             30                                                    0.10     95             66                                                    0.01     98             66                                                    ______________________________________                                    

Lamp tests carried out on La₀.80 Ce₀.20 Al₁₁ O₁₈ and CeAl₁₁ O₁₈confirmed the superior resistance of the lanthanum cerium aluminatephosphor composition to process degradation.

The efficiency of these phosphor compositions was assessed by comparingthem to (Ba,Mg) aluminate: Eu²⁺, which is a well-known efficient blueemitting phosphor. The integrated intensity of La₀.8 Ce₀.2 Al₁₁ O₁₈ wasfound to be virtually identical to that of the (Ba,Mg) aluminate: Eu²⁺.Further, it was found that the quantum efficiency varied little as afunction of x and y for La_(1-x) Ce_(x) Al_(y) O_(3/2)(y+1) in the range0<x<1.0, 11.0≦y≦13.5.

The following Examples are given to enable those skilled in this art tomore clearly understand and practice the present invention. They shouldnot be considered as a limitation upon the scope of the invention butmerely as being illustrative and representative thereof.

EXAMPLE I

A blended mixture was made of

1,1859 g CeO₂

4,7678 g La₂ O₃

29.558 g Al(OH)₃ --;

The mixture was precalcined in a 50 cc crucible at a temperature ofabout 850° C. for about one hour in air. After cooling, the precalcinedmixture was fired for 4 hours at a temperature of about 1750° C. in dryhydrogen. After cooling and pulverizing, the fired mixture was fired asecond time for four hours at a temperature of about 1750° C. in dryhydrogen. The twice fired mixture was cooled and pulverized and fired athird time for 4 hours at a temperature of about 1750° C. in dryhydrogen. After cooling and pulverizing, a material was obtained havinga composition represented by the formula

    La.sub.0.80 Ce.sub.0.20 Al.sub.11.0 O.sub.18.0.

EXAMPLE II

0.8189 g lanthanum (III) oxide was dissolved in 15ml hot 1:1 nitricacid. 0.2376g cerium (III) nitrate hexahydrate was dissolved in the hotlanthanum (III) oxide solution. The solution containing the lanthanum(III) oxide and cerium (III) nitrate hexahydrate was diluted to a totalvolume of 200 ml with distilled water. The diluted solution was heatedto a temperature of about 90° C. 3.1861g -aluminum oxide was added tothe about 90° C. solution with stirring to form a suspension. About 4gsolid oxalic acid was also added to the about 90° C. solution. Thesolution was stirred for 15 minutes; cooled to ambient temperature; andallowed to stand for a period of time sufficient for the precipitationreaction to occur (about 4 hours). The resulting mixture was stirred andfiltered. The precipitate was transferred to a beaker and dried at atemperature of about 140° C. The dried precipitate was then crushed andsieved through 100 mesh. The sieved precipitate was fired in an aluminacrucible in wet forming gas (5%H₂ /95%N₂) at a temperature of 1775° C.for three hours. The fired material was allowed to cool to ambienttemperature and was pulverized. The material obtained is represented bythe formula

    La.sub.0.90 Ce.sub.0.10 Al.sub.11 O.sub.18.

EXAMPLE III

0.8189g lanthanum (III) oxide was dissolved in 10 ml of hot 1:1 nitricacid. 0.2376g cerium (III) nitrate hexahydrate was dissolved in the hotlanthanum (III) oxide solution. The solution containing the dissolvedlanthanum (III) oxide and cerium (III) nitrate hexahydrate was dilutedto a total volume of 150 ml with distilled water. 3.1861g γ-aluminumoxide was added to the diluted solution with stirring to form asuspension. Ammonium carbonate was added until the solution was slightlybasic. The solution was stirred for 10 to 15 minutes. The solution wasallowed to stand for a period of time sufficient for the precipitationreaction to occur (about 16 hours). The resulting mixture was stirredand filtered. The precipitate was transferred to a beaker and dried at atemperature of about 140° C. for 3 hours. The dried precipitate wascrushed to a fine powder in a mortar and pestle. The crushed precipitatewas fired in an alumina crucible in wet forming gas (5%H₂ /95%N₂) at atemperature of 1775° C. for 3 hours. The fired material was allowed tocool to ambient temperature and was pulverized. The material obtained isrepresented by the formula

    La.sub.0.90 Ce.sub.0.10 Al.sub.11 O.sub.18.

While there has been shown and described what is at present consideredthe preferred embodiments of the invention it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention as defined by the appendedclaims.

What is claimed is:
 1. A luminescent phosphor composition consistingessentially of lanthanum cerium aluminate phosphors represented by theformula

    La.sub.1-x Ce.sub.x Al.sub.y O.sub.(3/2) (y+1)

wherein 0.10 <x <1 and 11.0 ≦y ≦12.5 said lanthanum cerium aluminatephosphors existing as a magnetoplumbite/-beta-alumina single phasematerial characterized by ultraviolet peak emission and blue peakemission with relative intensities of the ultraviolet peak emission andthe blue peak emission being a function of x and y, and furthercharacterized by predominantly blue peak emission.
 2. A white emittingphosphor blend which contains a blue emitting phosphor wherein the blueemitting phosphor is the phosphor composition of claim
 1. 3. Theluminescent phosphor composition of claim 1 wherein 0.10<x<1.0 and11.0≦y≦12.
 4. A white emitting phosphor blend which contains a blueemitting phosphor wherein the blue emitting phosphor is the phosphorcomposition of claim
 3. 5. An electrical discharge device comprising:a.a pair of electrodes; b. a glass envelope disposed about and sealed tosaid electrodes; c. a fill of mercury disposed within said envelope; andd. a coating of a luminescent phosphor composition, consistingessentially of lanthanum cerium aluminate phosphors represented by theformula:

    La.sub.1-x Ce.sub.x Al.sub.y O.sub.(3/2) (y+1)

wherein 0.10 <x <1.0 and 11 ≦y ≦12.5, said lanthanum cerium aluminatephosphors existing as a magnetoplumbite/beta-alumina single phasematerial characterized by ultraviolet peak emission and blue peakemission with relative intensities of the ultraviolet peak emission andthe blue peak emission being a function of x and y.